1. Field of the Invention
The field of the invention is digital controllers for controlling a machine, an assembly line or a commercial process. More particularly, the invention relates to programming terminals, distributed processing systems and communication networks.
2. Description of the Background Art
Programmable controllers are used in many industrial and commercial settings to control the operation of various machines and processes. They fall into the broad category of real-time sampled data systems. In a certain maximum time, inputs are read, controlling equations are applied, and actuator outputs are provided.
In U.S. Pat. No. 4,477,882, a group of small programmable controllers are connected in a ring network. A control program is set up and loaded into each respective controller.
While the programs are executing, the controllers communicate I/O image data, corresponding to the state of their inputs and outputs, in round-robin fashion. If eight controllers are in the ring, each controller puts its I/O image data on the network in one of the eight time slots. During the other seven time slots, each controller relays a set of image data from a neighboring controller on one side to a neighboring controller on the other side. The controllers each maintain an image of its own I/O image data as well as I/O image data for the other seven controllers in the ring. The update rate for each controller is limited to eight intervals which are required for all of the I/O image data to be communicated around the ring.
Another approach to communication of I/O data is disclosed in U.S. Pat. No. 4,897,834. There each network node broadcasts its input data on a repetitive basis. The other nodes are programmed to recognize the source address of the sending node and a bit of input data in the broadcast, and to control an output in response to this input data. However, the only type of control that can be exercised is a 1-to-1 mapping. If an input bit is true, then the output can be set true, or the NOT function can be performed: if an input bit is false, the output can be set true. There are no combinational logic functions performed at the I/O nodes. An output cannot be controlled in response to the AND function for two bits of input data.
Also, in U.S. Pat. No. 4,897,834, the mapping of the I/O nodes is accomplished by a direct connection of a programming device to each node. There is no downloading of mapping information through the broadcast network.
Flood et al., U.S. Pat. No. 4,937,777, discloses distribution of ladder control programs to separate processors connected through a rack backplane. Execution of these programs is linked through description files generated by a high-level sequential function chart program. I/O status data is collected by I/O scanners and is accessed in an image table, as needed, by the ladder control programs. The distribution of the ladder logic is essentially manual, the ladder program is set up first and then assigned to a controller by entering a controller ID number in a step of the sequential function chart.
All of these prior systems lacked the ability to distribute subdivisions of a ladder diagram program for execution at different nodes on a serial data network.